Synthesis, characterization and photoelectrochemical properties of poly(3,4-dioctyloxythiophene)–CdS hybrid electrodes

doi:10.1016/j.electacta.2007.11.009

Electrochimica Acta

Volume 53, Issue 6, 15 February 2008, Pages 2984-2993

https://doi.org/10.1016/j.electacta.2007.11.009 Get rights and content

Abstract

CdS–poly(3,4-dioctyloxythiophene) (CdS–PDOT) hybrid electrode has been prepared by electrosynthesis of PDOT on Au substrate followed by electrodeposition of Cd and its chemical transformation into CdS. The polymer and semiconductor obtained by this method form hemispherical structures dispersed on the substrate. The synthesized composites were characterized by UV–vis absorption spectra and energy dispersive X-ray spectra (EDS). The AFM images of the electrodes covered with different amounts of each component were correlated with photoactivity of the hybrid electrodes. Photoresponses of Au/PDOT–CdS electrodes under illumination in aqueous solution of Na₂S were also compared with those of CdS without polymer. Enhancement of the photocurrent achieved for some polymer-to-semiconductor ratio is discussed in terms of the hybrid electrode morphology and hole-mediating properties of PDOT. The power conversion efficiency of the device based on CdS–PDOT hybrid electrode was determined from photocurrent–potential behavior of two electrode system, Au/CdS–PDOT/0.1 M Na₂S/Pt with a variable resistance in series in the external circuit.

Introduction

Conducting polymer–semiconductor hybrids represent a novel class of materials for low-cost photovoltaic devices. Combining of p-type conducting polymers and n-type semiconductors is advantageous for separation of the charges generated under illumination due to a high electron affinity of inorganic semiconductor and relatively low ionization potential of the polymer [1], [2]. This allows for transport of the charge carriers in the separate materials with a low probability of recombination.

The polymer–semiconductor composites are usually prepared in the form of bilayers of the type ITO (or Au)/conducting polymer/semiconductor/metal. The ITO or Au substrates may be covered with the conducting polymer by electropolymerization or spin-casting a solution of the polymer soluble in chloroform. Semiconductor can be synthesized as a film by chemical bath deposition [3], self-assembling [4] or deposited in the form of dispersion of nanoparticles in soluble conducting polymer [2], [5], [6], [7]. The latter method is advantageous because of the increase of p–n junction interface and in effect the increase of photovoltaic efficiency. However, the semiconductor nanocrystals are not easily dispersed in conjugated polymer. A high surface energy of nanocrystals leads to their aggregation. The most efficient method of avoiding this effect is stabilization of nanocrystals by surrounding surfactant. According to the literature, the hybrid device consisting of a polymer layer and monodispersed CdS capped with thioglycerol, spin coated on the polymer film possesses a dual properties of photocurrent generation and electroluminescence [8]. However, the surfactant usually tends to isolate the semiconductor from conducting polymer phase leading to enhanced electron–hole recombination and decrease of quantum efficiency of the light energy conversion.

Another way of fabrication of semiconductor nanocluster–polymer composites is co-deposition of the two components [9]. The semiconductor nanoparticles may be also covalently linked to the monolayer assembled on Au electrode [10], [11], [12].

Preparation of the polymer–semiconductor blends is relatively easy but control of amount of two composites is problematical. In the present work we demonstrate a simple method of preparation of poly(3,4-dioctyloxythiophene)–CdS composite for photovoltaic cells. Polymer is obtained by electrodeposition, whereas CdS is formed by electrochemical/chemical synthesis. Electrochemical methods offer a wide range of control of the polymer and CdS amounts. In this work we optimize the conditions to obtain the hybrid electrode of enhanced photoactivity in comparison with that of pure CdS.

Section snippets

Experimental

All electrochemical measurements reported in this paper were performed by means of Autolab (EcoChemie, The Netherlands) in a three electrode cell with a platinum gauze counter electrode, Ag/AgCl, Cl⁻ (std., aq.) reference electrode and Au working electrode. The Au substrate was polished to a mirror finish with alumina slurry and washed in an ultrasonic bath.

Procedure of synthesis and characterisation 3,4-dioctyloxythiophene (DOT) have been described elsewhere [13]. All other chemicals, LiClO₄

Electrochemical behavior of Au/PDOT electrodes in aqueous solutions of 0.1 M LiClO₄ and in 0.1 M Na₂S

Poly(3,4-dioctyloxythiophene) (PDOT) was deposited on Au electrode from acetonitrile solution of 10 mM monomer + 0.1 M LiClO₄ by cycling within the potential range from 0 to 1.34 V at the scan rate 100 mV s⁻¹. Exemplary electropolymerization curves are presented in Fig. 1.

As visible in the inset in Fig. 1, the PDOT is electroactive not only in acetonitrile (as most of polythiophenes) but also in aqueous solution, owing to the presence of oxygen in the side groups [13]. This feature is promising in

Conclusions

Au/poly(3,4-dioctyloxythiophene)–CdS hybrid electrodes have been prepared by means of four-step electrochemical/chemical method. Au was firstly decorated with the polymer by cyclic voltammetry and then, CdS was formed by potentiostatic deposition of Cd followed by its chemical transformation into Cd(OH)₂ in NaOH and finally to CdS by immersion in Na₂S. The CdS crystals obtained by this procedure are cadmium rich with Cd/S ratio from 2.5 to 3.

CdS and PDOT are deposited in the form of

Acknowledgements

Support for this work by the Polish Ministry of Education and Science under grants 3T08A04128 and N20409131/2142 is gratefully acknowledged.

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Synthesis, characterization and photoelectrochemical properties of poly(3,4-dioctyloxythiophene)–CdS hybrid electrodes

Abstract

Introduction

Section snippets

Experimental

Electrochemical behavior of Au/PDOT electrodes in aqueous solutions of 0.1 M LiClO4 and in 0.1 M Na2S

Conclusions

Acknowledgements

Solar Energy Mater.

Thin Solid Films

Thin Solid Films

Electrochim. Acta

J. Electroanal. Chem.

J. Electroanal. Chem.

J. Electroanal. Chem.

J. Electroanal. Chem.

Electrochim. Acta

J. Electroanal. Chem.

Solar Energy Mater. Solar Cells

Phys. Rev. B

Phys. Rev. B

J. Am. Chem. Soc.

Electrochemical behavior of Au/PDOT electrodes in aqueous solutions of 0.1 M LiClO₄ and in 0.1 M Na₂S